Development of the high chromium ferritic steel FBR grade (2). Mechanical properties and observations of as received and aged some trial products controlling V and Nb contents

The high chromium (Cr) ferritic steels have both excellent thermal properties and strength at elevated temperature. Therefore, the applicability of high Cr ferritic steels for structural material of the future advanced Fast Breeder Reactor is investigated. In previous study, a series of trial products controlling the vanadium (V) and niobium (Nb) contents is produced to investigate the efficiency and/or stability of these elements for the development of the high chromium ferritic steel FBR grade. In this study, tensile, impact, creep tests and metallurgical observations are conducted in order to investigate the influence of V and Nb contents on the mechanical properties and stability of microstructures. These tests and observations are conducted for the as received materials and aged ones. As a result, the following conclusions are obtained; (1) The higher strength and the lower ductility are obtained with the increases of V and Nb as far as this study investigates. There is little influence of V and Nb on relation between steady state creep rate, time to beginning of tertiary creep and creep rupture time. (2) The impact properties degrade with the increases of V and Nb. On the other hand the impact properties hardly degrade with thermal aging at 600degC for 6000h. It is thought that reasons are inference of hardly deposit of Laves phase. (3) Fine MX (V(C, N) and/or NbC) precipitates are observed in the steels which contain V and/or Nb. The number of MX precipitates increases with the increase of contents of V and/or Nb. It is show that they are likely to be stable under the condition of aging at 600degC for 6000h. (4) The fine Cr2(C, N) precipitates is nucleated from V(C, N). And the fine Cr2(C, N) may be expected to suppress the growth of M23C6. This may be caused by V contained in Cr2(C, N). (author)